American scientists Stanley Miller and Harold Urey used an apparatus similar to the one in the diagram to simulate how life could have formed on Earth. The apparatus contained hydrogen gas, water vapor, ammonia, and methane. These gases were subjected to a spark, to simulate lightning. The Miller-Urey experiment showed that, under certain conditions, organic compounds could form from inorganic molecules. How did this, as well as other scientists’ contributions help explain the origin of life on Earth? (SC.912.L.15.8) A. Scientists think that life could not have developed through natural chemical and physical processes. B. The experiment proved that methane and ammonia will always give rise to organic molecules in any circumstance. C. Scientists think it is possible that organic compounds formed from the inorganic compounds present on Earth billions of years ago. D. The experiment used the exact inorganic compounds present on Earth billions of years ago and left little doubt about the mechanism of early life.

an individual’s sex (i.e., whether they are a male or female) is determined by the sex chromosomes. most people have two sex chromosomes, one that is inherited from their mother and one that is inherited from their father. typically, females have two x chromosomes (xx) and males have one x chromosome and one y chromosome (xy). conditions caused by changes (“mutations”) in genes located on the x chromosome are considered x-linked.

x-linked recessive inheritance

most x-linked conditions are recessive. this means that in a person with two x chromosomes (most females), both copies of a gene (i.e., one on each x chromosome) must have a change or mutation whereas in a person with one x chromosome (most males), only one copy of a gene must have a mutation. a female with a mutation in one copy of a gene on the x chromosome is said to be a “carrier” for an x-linked condition. a male with a mutation in a gene on the x chromosome is typically affected with the condition. because females have two copies of the x chromosome and males have only one x chromosome, x-linked recessive diseases are more common among males than females. however, x-linked recessive diseases can occur in both males and females.

for x-linked recessive disorders, an unaffected carrier mother who has a mutation in a gene on the x chromosome can transmit either the x chromosome with this mutation or a “normal” x chromosome to her children. if the father is unaffected, none of her daughters will be affected and all of her daughters will be unaffected—since they will inherit at least one normal x chromosome from their father. however, each daughter will have a 50% chance of being an unaffected carrier like her mother and a 50% chance of both x chromosomes being normal.

high growth (apex)